1. Field of the Invention
This invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube which includes a panel with a substantially rectangular effective portion whose outside face is almost flat or slightly curved and whose inside face has a curvature in the direction of minor axis near the short sides of the portion and which thereby improves the flatness of images appearing on the effective portion.
2. Discussion of the Background
Generally, a color cathode ray tube includes a vacuum enclosure made of a glass panel and a glass funnel. The effective portion of the glass panel on which images are displayed is substantially rectangular. On the inside face of the effective portion of the panel, a fluorescent substance screen composed of a three-color fluorescent substance layer is provided. On the inside, a substantially rectangular shadow mask is provided so as to face the fluorescent substance screen. The shadow mask is composed of a mask body and a mask frame provided on the periphery of the mask body. The mask body is such that a large number of electron beam passing holes have been made in a substantially rectangular effective surface.
In the color cathode ray tube with such a configuration, three electron beams emitted from an electron gun provided in the neck of the funnel are deflected by a deflection unit on the outside of the funnel toward the fluorescent substance screen through the shadow mask. The electron beams scan the fluorescent substance screen horizontally and vertically, thereby producing a color image on the fluorescent substance screen.
In recent years, effort has been directed toward flattening the image display section of the color cathode ray tube to improve the visibility of images. Specifically, in the color cathode ray tube, the inside and outside faces of the effective portion of the panel are generally fattened. Since the clearance (q value) between the inside face of the panel's effective portion and the effective surface of the shadow mask facing the fluorescent substance screen provided on the inside face of the effective portion must be maintained within specific permitted limits, the effective surface of the shadow mask is also required to be flattened. Flattening the effective surface of the shadow mask introduces the danger of decreasing the strength with which the curved surface of the shadow mask is retained. Therefore, even when the outside face of the effective portion of the panel has been flattened, its inside face has been often left curved.
One example of such a color cathode ray tube has been disclosed in Japanese Patent Application No. 8-49030 (Japanese Laid Open Patent Publication No. H9-245685). In the application, the following panel of a color cathode ray tube has been proposed. In the panel, the outside face of its effective portion is made almost flat. The inside face of its effective portion is formed into a cylindrical curved surface, with the radius of curvature in the direction of major axis of the inside face being almost infinite and the radius of curvature in the direction of minor axis being almost constant. In the color cathode ray tube where the inside face of the effective portion of the panel has a cylindrical curved surface, the effective surface of the shadow mask located inside is also formed into a cylindrical curved surface having a curvature in the direction of minor axis so as to correspond to the inside face of the effective portion of the panel.
In a color cathode ray tube the effective surface of whose shadow mask is a cylindrical curved surface having a curvature in the direction of minor axis and which has a black stripe fluorescent substance screen composed of a striped black light absorption layer, each stripe long and narrow in the direction of minor axis of the panel, and a striped three-color fluorescent substance layer provided so as to fill up the spacing of the light absorption layer, making the curvature in the direction of minor axis larger than that in the direction of major axis helps improve the curved-surface retaining strength. Specifically, in the black stripe color cathode ray tube, electron beam passing holes in the effective surface of the shadow mask are arranged in the direction of minor axis. The slit-like electron beam passing holes are arranged in a line via a narrow bridge. Lines of electron beam passing holes extending in the direction of minor axis are arranged with a specific arrangement pitch in the direction of major axis. Therefore, when the flatness of the shadow mask is almost the same, making the curvature in the direction of minor axis than that in the direction of major axis helps improve the strength with which the curved surface is retained. Such a structure is particularly effective for a wide tube with an aspect ratio of 16:9.
Furthermore, the following configuration has been proposed for a panel and a shadow mask suited to flatten the image display section. For the inside face of the effective portion of the panel, the radius of curvature in the direction of major axis is made almost infinite near the center of the panel and almost constant on the periphery of the panel. The radius of curvature in the direction of minor axis is made larger near the center of the panel than on the periphery of the panel. When the effective surface 2 of the shadow mask 1 is formed into a curved surface that corresponds to the inside face of the panel as shown in FIG. 1, it is possible to improve the strength with which the curved surface of the shadow mask is retained.
Since an image appears on the fluorescent substance screen provided on the inside face of the effective portion of the panel, the effect of the thickness difference on an image viewed through the effective portion is small in a color cathode ray tube with a small difference in thickness between the center of the effective portion and its periphery. In the case of a panel flattened as described above, the thickness difference between the center of the effective portion and its periphery is greater, increasing the effect of the thickness difference on an image viewed through the effective portion. For an image to appear on a truly flat rectangular plane, the inside and outside faces of the effective portion need to be flat.
Specifically, the flatness of an image appearing on the effective portion of the panel is influenced by not only the shape of the outside face of the effective portion but also the shape of its inside face. In the case of a panel whose image display section has been flattened, a rectangular screen drawn on the fluorescent substance screen formed on the inside face of the effective portion looks like a barrel with the short sides projecting outward from the center of the effective portion, when viewed from the front of the effective portion on the central axis. The tendency is more prominent when the screen is viewed form the side. As a result, the screen does not take the form of a truly rectangular plane and looks round. This prevents flat images from being produced as expected, making no sense to flatten the image display section.
As described above, the image display sections of color cathode ray tubes have recently been flattened to improve the visibility of images. In this connection, the following panel has been proposed. In the panel, the outside face of the effective portion of a panel on which a fluorescent screen is provided is made almost flat. Its inside face is formed into a cylindrical curved surface, with the radius of curvature in the direction of major axis of the inside face being made almost infinite and the radius of curvature in the direction of minor axis being made almost constant. Alternatively, the inside face is formed into a curved surface with the radius of curvature in the direction of major axis being made almost infinite near the center and almost constant on the periphery and the radius of curvature in the direction of minor axis being made larger near the center than on the periphery. In such a panel, even when the outside face of the effective portion is almost flat, the inside face has a curvature in the direction of minor axis at least near the short sides. When a screen drawn on the rectangular fluorescent substance screen formed on the inside face of the effective portion is viewed from the front of the effective portion on the central axis, the short sides looks as if they projected outward from the center of the effective portion. The tendency is more prominent, when the image is viewed from the side. As a result, the screen does not take the form of a truly rectangular plane and looks round. This prevents flat images from being produced as expected, which makes no sense to flatten the image display section.